On Mon 12-01-26 14:37:49, Mathieu Desnoyers wrote: > On 2026-01-12 03:42, Michal Hocko wrote: > > Hi, > > sorry to jump in this late but the timing of previous versions didn't > > really work well for me. > > > > On Sun 11-01-26 14:49:57, Mathieu Desnoyers wrote: > > [...] > > > Here is a (possibly incomplete) list of the prior approaches that were > > > used or proposed, along with their downside: > > > > > > 1) Per-thread rss tracking: large error on many-thread processes. > > > > > > 2) Per-CPU counters: up to 12% slower for short-lived processes and 9% > > > increased system time in make test workloads [1]. Moreover, the > > > inaccuracy increases with O(n^2) with the number of CPUs. > > > > > > 3) Per-NUMA-node counters: requires atomics on fast-path (overhead), > > > error is high with systems that have lots of NUMA nodes (32 times > > > the number of NUMA nodes). > > > > > > The approach proposed here is to replace this by the hierarchical > > > per-cpu counters, which bounds the inaccuracy based on the system > > > topology with O(N*logN). > > > > The concept of hierarchical pcp counter is interesting and I am > > definitely not opposed if there are more users that would benefit. > > > > From the OOM POV, IIUC the primary problem is that get_mm_counter > > (percpu_counter_read_positive) is too imprecise on systems when the task > > is moving around a large number of cpus. In the list of alternative > > solutions I do not see percpu_counter_sum_positive to be mentioned. > > oom_badness() is a really slow path and taking the slow path to > > calculate a much more precise value seems acceptable. Have you > > considered that option? > I must admit I assumed that since there was already a mechanism in place > to ensure it's not necessary to sum per-cpu counters when the oom killer > is trying to select tasks, it must be because this > > O(nr_possible_cpus * nr_processes) > > operation must be too slow for the oom killer requirements. > > AFAIU, the oom killer is executed when the memory allocator fails to > allocate memory, which can be within code paths which need to progress > eventually. So even though it's a slow path compared to the allocator > fast path, there must be at least _some_ expectations about it > completing within a decent amount of time. What would that ballpark be ?
I do not think we have ever promissed more than the oom killer will try to unlock the system blocked on memory shortage. > To give an order of magnitude, I've tried modifying the upstream > oom killer to use percpu_counter_sum_positive and compared it to > the hierarchical approach: > > AMD EPYC 9654 96-Core (2 sockets) > Within a KVM, configured with 256 logical cpus. > > nr_processes=40 nr_processes=10000 > Counter sum: 0.4 ms 81.0 ms > HPCC with 2-pass: 0.3 ms 9.3 ms These are peanuts for the global oom situations. We have had situations when soft lockup detector triggered because of the process tree traversal so adding 100ms is not really critical. > So as we scale up the number of processes on large SMP systems, > the latency caused by the oom killer task selection greatly > increases with the counter sums compared with the hierarchical > approach. Yes, I am not really questioning the hierarchical approach will perform much better but I am thinking of a good enough solution and calculating the number might be just that stop gap solution (that would be also suitable for stable tree backports). I am not ruling out improving on top of that by a more clever solution like your hierarchical counters approach. Especially if there are more benefits from that elsewhere. -- Michal Hocko SUSE Labs
